1. Field of the Invention
The present invention relates generally to improving weight measurements of thermogravimetric analyzers. More particularly, the present invention relates to improving weight measurements for vertical thermogravimetric analyzers with a balance positioned above the furnace.
2. Background of the Invention
Thermogravimetry is an analytical technique wherein a sample to be evaluated is subjected to a desired temperature program while its weight and temperature are measured. The weight change or the rate of weight change with respect to time or temperature may be displayed as a function of the measured temperature or of time and various evaluations may be performed. The weight curve versus temperature may be analyzed to determine the magnitude of weight changes that occur and the temperature or the range of temperatures at which they occur or other more sophisticated analyses may be performed such as those that determine the kinetics of the process responsible for the weight change.
A typical thermogravimetric analyzer (TGA) consists principally of a sensitive balance to weigh the sample dynamically and a furnace to heat the sample. TGAs are described, for example, in U.S. Pat. No. 5,165,792, which is incorporated by reference herein in its entirety. There are generally three configurations for TGAs: a horizontal furnace 901 with a balance 903 alongside the furnace, as shown in FIG. 9A, a vertical furnace 905 with the balance 903 below the furnace 905, as shown in FIG. 9B, or a vertical furnace 905 with the balance 903 above the furnace, as shown in FIG. 9C. In each of the configurations, a pan 907 is connected to the balance 903 by a support 909 . In principle, any type of balance may be employed. However, the majority of TGAs use a null type balance that measures the force required to maintain the balance in an equilibrium position. (An overview of the different types of balances that may be employed in a TGA may be found in “Automatic and Recording Balances,” Saul Gordon and Clement Campbell, Anal. Chem. 32(5) 271R-289R, 1960.)
A null balance comprises a drive system that applies force to the balance movement that supports the sample pan and tare pan or counterweight, a displacement sensor, and electronic control and measurement circuitry. These drive systems are typically electromagnetic drive systems. In operation, a force applied to the balance movement by the drive system maintains the balance in the equilibrium, or null position. The force applied by the drive system to maintain the null position is a measure of the sample weight. Changes in sample weight cause the balance to be displaced from the equilibrium position, the displacement is sensed by the sensor and the balance movement is returned to equilibrium by the drive system. Null balances are capable of sensing mass changes that are well below one microgram. Null balances are also robust and relatively inexpensive.
When attempting to make high sensitivity weight measurements a number of undesirable forces may act on the balance, sample, sample pan, and associated components of the weighing system. (Some of these undesirable forces are described in Ultra Micro Weight Determination in Controlled Environments, S. P. Wolsky and E. J. Zdanuk eds., 1969, Interscience, 39-46). For example, adsorption and desorption from the moving components of the balance may cause spurious weight changes. Temperature fluctuations within the balance may cause weight changes due to thermal expansion of the balance arms or may affect the strength of the field developed by permanent magnets in electromagnetic drive systems so equipped. Static electric charges may collect and act on the balance assembly, sample, and pan. Convection currents within the furnace or within the balance chamber may generate forces on the sample pan and associated components. Buoyancy forces that vary with changes in gas density act on the sample and pan within the furnace. Radiometric forces that result from thermomolecular flow may act on the balance components in regions where temperature gradients exist. This particular problem may be especially severe when operating under vacuum. The dynamic weight baseline performance of a TGA depends critically on minimizing or compensating for these undesirable forces or disturbances.
The dynamic weight baseline of a TGA is the weight measured when an experiment is performed without a sample. In principle the dynamic weight measurement should be zero regardless of the temperature or the heating rate of the instrument. Deviations from zero are the result of disturbances acting on the balance assembly or pan. Given that a TGA is used to measure changes in weight that occur as a function of temperature or time, any weight change that occurs in the absence of a sample introduces uncertainty in the weight change measured during an experiment when a sample is present.
The choice of TGA configuration can affect the degree to which these extraneous forces act on the weight measurement. The horizontal configuration, as shown in FIG. 9A, is largely immune to convection currents and thermomolecular forces because they act on the weighing system orthogonally to the gravitational force acting on the sample and the balance. Horizontal systems, however, generally have much heavier balance components that act to reduce sensitivity. This is because the sample and its pan must be supported by a cantilever beam structure capable of resisting high temperatures within the furnace. The heavier structure requires a more robust suspension that can also reduce the balance sensitivity. Thermal expansion of the cantilever can materially affect the weight measurement, and although in principle it can be readily compensated for, thermal expansion remains a significant potential source of weighing errors.
Vertically arranged TGAs may have the balance below or above the furnace, as shown in FIGS. 9B and 9C. When the balance is below the furnace, as shown in FIG. 9B, the structure that supports the sample pan must be relatively massive because it supports the pan in compression and must resist the tendency to buckle, a tendency that is exacerbated by the high temperatures that are often achieved in TGA. Like the horizontal TGA, it requires a robust suspension and has relatively low sensitivity. However, it is easier to isolate the balance thermally from the furnace because hot gas and effluent from the furnace tend to rise because their temperature is high and density is low.
A TGA using a vertical furnace with the balance above it, as shown in FIG. 9C, offers the highest sensitivity because the mass of the pan suspension can be minimized, requiring only a fine filament to suspend the pan. This allows the mass of the balance and its suspension to be minimized. However, thermal isolation is more difficult because of the tendency of hot gas and effluents to rise. A vertical TGA is more susceptible to convection effects because of the orientation of the furnace and the relatively large vertical thermal gradients that accompany this configuration, and also because the thermomolecular forces act in parallel to the force of gravity and thus affect the weight measurement directly.